1. Field of the Invention
This invention relates to vibration sensors and, more particularly to sensors for detecting fuel detonation ("knock") in an internal combustion engine.
2. Description of the Prior Art
It is well known that fuel within the combustion chambers of gasoline internal combustion engines should burn without detonation. The sound of the detonation, known as engine knock, is annoying and the higher cylinder pressures associated therewith can damage the engine. Systems employed to eliminate engine knock generally have means that detect the occurence of the knock and then automatically adjust the ignition spark timing. Knock detecting means used in these systems have transducers composed either of piezoelectric material or magnetostrictive material.
Knock detector devices using piezoelectric transducers are expensive because they use crystals or ceramics that must be specially manufactured and prepared to obtain proper operation. Furthermore, piezoelectric devices are charge type devices having high internal impedance. There can be significant attenuation of the output signal within even a short length of leadout wiring, and it is necessary to locate a signal amplifier having low input impedance near to the piezoelectric device. The need for such preamplification, in turn, raises the cost of the knock detection system. In addition, piezoelectric materials are brittle. They crack easily when subject to compressive stresses and are not readily constructed using clamping type devices. Instead, they must be adhesively bonded to a metal substrate to provide structural reinforcement and avoid breakage. The adhesives in turn can be heat sensitive and can experience aging problems.
Knock detector devices using magnetostrictive transducers employ crystalline metal alloys which require special annealing procedures and which even then have low magnetomechanical coupling (MMC) factors (in the order of 0.25). Thus, more transducer material is needed to produce a given output signal. Such transducer materials are usually soft mechanically. They are easily scratched and deformed, and crystalline metal magnetostrictive transducer materials having higher MMC factors are expensive and brittle. As a result, there can be undesirable limitations on how these materials are handled during manufacturing operations. In addition, a high magnetic bias field in the order of 300 oersteds is needed for proper operation of present devices. This requires the use of more expensive and powerful magnets to impart the levels magnetization needed for these materials, and if electromagnets are used to provide this magnetic bias, those electromagnets will require more electrical power and can cause heat dissipation problems. Furthermore, crystalline metal alloys employed as magnetostrictive transducers are negative magnetostriction alloys and require substantial amounts of material to support the compressive loads to which they must be subjected during operation. Mechanical structures needed to provide those compressive loads are not easily changed to make the structures mechanically resonant with the different vibrational knock frequencies that are characteristic of different engine models. Since each engine model can have a different vibrational frequency characteristic of knock, many different knock detectors must be specifically manufactured and inventoried to ensure that a matched detector, tuned to resonate at the characteristic frequency, is available for each particular engine model. For these reasons, knock detectors of the type described above have more complex and expensive structures or provide output signals of lower strength than are desireable.